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A toxic glycoside derived from the seeds of certain African plants, such as Strophanthus gratus, that has been used historically as a drug to stimulate contractions of heart muscle in treating heart failure and other conditions. It specifically blocks the action of sodium/potassium ATPase (see sodium pump), a crucial membrane-bound transport protein that maintains differential ion concentrations between a cell's interior and the external environment. For this property, ouabain is used widely in biomedical research, and it is now known to occur naturally in the body at very low concentrations.